Stocks slumped and bonds rallied on heightened concern that the spread of the novel coronavirus will slam global economic growth.

The S&P 500 index erased its 2020 gain and the Dow Jones industrial average dropped more than 600 points as traders remained on edge over the impact of the disease that’s now infected about 10,000 people around the world. A gauge of U.S. equity volatility spiked to an almost four-month high. Caterpillar slid as its outlook trailed estimates, adding to worries about global business spending. Amazon, however,  soared after a blowout quarter. Treasury 30-year yields breached 2% for the first time since October.

Investors in China will get their first chance to trade since Jan. 23 as financial markets reopen Monday. For equities, the declines are likely to be exacerbated by the amount of leverage in the market. That could create a downward spiral where steep losses become steeper with traders facing margin calls.

As an example of how extreme selling can be, the Shanghai equity benchmark fell almost 6% in May, when it resumed trading following a holiday break on negative trade-war news.

The final week of January was tumultuous across global markets as a barrage of corporate earnings, central-bank decisions and economic data landed in the growing shadow of the epidemic.

The outbreak is forecast to cut U.S. economic growth by 0.4 percentage point in the first quarter as the number of tourists from China plunges and exports to the Asian nation slow, according to Goldman Sachs Group. Still, a report showed that American consumer sentiment increased in January to an eight-month high, indicating sustained optimism in the face of the novel coronavirus.

“The virus outbreak represents this unknown that, frankly, markets aren’t very good at handicapping,” said David Lafferty, chief market strategist at Natixis Investment Managers in Boston. “It’s almost like an open-ended risk.”

American stocks posted their worst month since August, with the S&P 500 dropping more than 3% from its all-time high on Jan. 17. Energy and raw-material companies led losses in the U.S. equity measure this month, while utilities, tech and real-estate shares advanced.